This invention relates generally to the in situ gasification of coal to produce a combustible gas product.
More specifically, this invention relates to methods and techniques for conducting in situ coal gasification on a practical and commercial scale.
Attempts to develop in situ coal gasification technology have occurred around the world during the last 60 years. Large research efforts have been undertaken in the U.S., the USSR, the U.K., France, Poland, Czechoslovakia, Canada, the Federal Republic of Germany and Belgium. Only in the USSR has the technology operated at commercial scale.
Impending shortages of natural gas and petroleum liquids together with sharply increasing prices for those commodities has focussed renewed interest on all processes which hold promise for the practical conversion of coal into gaseous and liquid forms. In situ coal gasification is one of the more highly developed techniques but commercial practicability in this country has yet to be convincingly demonstrated.
Successful application of in situ coal gasification technology results in recovery of gaseous products and liquid byproducts from coal resources which cannot be recovered using conventional coal mining techniques. Either low- or intermediate- Btu gas can be obtained from the process depending upon whether air or oxygen is the injected oxidant, respectively. The process has several apparent advantages over surface-based coal gasification operations in that the coal need not be mined, no coal transportation or preparation is required, the need for surface pressure vessels for gasification is eliminated, and solid waste disposal requirements are greatly reduced since the great majority of the ash is left underground. Less apparent but important advantages over surface-based gasification processes include: Increased thermal efficiency since the in situ gasifier can be operated at higher temperatures because concerns about corrosional and erosional effects of components in the product gas are reduced; lower high quality water requirements since water of any quality present in the coal seam or adjacent aquifers can serve as the hydrogen source required for gasification thereby lowering steam injection requirements; and, less sensitivity to economics of scale since the in situ gasification production facility consists of adding process wells to increase output with the cost of each well being roughly the same whether 100 or 1,000 wells are required.
The process is basically a simple one involving the following steps: Drilling and completing wells using conventional techniques in order to access the coal seam; enhancing the natural permeability of the coal seam in order to allow injection of sufficient oxidant to achieve efficient gasification conditions; and, gasification of the coal seam between successive pairs of process wells over a large area to provide the desired quantity of product output. Experiments have been conducted in the USSR on coals ranging in rank from lignites to anthracite in seams of variable thickness with dip angles from 0.degree. to near 90.degree. from horizontal.
The U.S. patent literature is replete with various in situ methods for recovering energy from coal. In spite of this plethora of prior art, there is lacking an appreciation of the practical economic and technical limits imposed by in situ operations and of the need for a method amenable to large scale systematic expansion of the process.
One common thread that runs explicitly or implicitly through much of the technical literature on in situ gasification is the criticality of the linkage path location between wells; that the linkage path must be located near the bottom of the coal seam to achieve a successful operation. Experimental support for this conclusion appears to be substantially based on the highly successful test burn at Hanna, Wyo., in 1976. Downhole instrumentation showed that the reverse combustion linkage path was located about 5 feet above the bottom of the 30-foot coal seam being gasified.
Later experimental tests have shown that linkage path location at or near the bottom of the seam does not guarantee success. The first of these tests, conducted at a site near Gillette, Wyo., in 1977, resulted in formation by reverse combustion of a linkage path 8 feet off the bottom of the 25-foot thick coal seam being gasified. The results were still disappointing during the subsequent gasification phase. These lower than expected results were due to unsuitable site characteristics rather than to the location of the linkage path. The lower than expected results have been explained by the conducting organization as the result of combustion zone override to the top of the seam due to blockage of the linkage path by roof collapse.
In the second test, also conducted near Gillette, Wyo., in 1979, directional drilling was utilized to place a small-diameter pathway in the lower 1/2 of the same 25-foot thick coal seam. After vertical wells were drilled and connected to the drilled pathway, reverse combustion was utilized to enlarge the drilled pathway. Again, the results were not up to expectations due to unsuitable site characteristics.
Conversely, location of the linkage pathway at or near the top of the seam does not preclude successful operations. The first test conducted at a site near Hanna, Wyo., in 1973 and early 1974 was successful even though later drilling of the affected area clearly showed that linkages created by reverse combustion were located in the top few feet of the 30-foot thick coal seam being used.
The inventors herein have found that the emphasis accorded linkage path location by the prior art has been misplaced; that, in fact, location of the linkage path is of no importance in the successful conduct of large-scale in situ gasification operations.